All tissues contain stromal and parenchymal cells and tissue-resident macrophages, which are critical for tissue homeostasis, repair and immunity. Macrophage colony stimulating factor is a critical cytokine for macrophage growth, differentiation, and development, and its deficiency leads to substantial macrophage deficits in the CSF1op/op mouse. While stromal cell-secreted CSF1 is known to be upregulated in response to various cytokines and hormones, very little is understood about how it may be regulated locally by interactions with macrophages, the primary MCSF consumer. Less is understood about how tumor cells can manipulate MCSF synthesis, except for a putative EGF-CSF1 paracrine positive feedback loop between macrophages and invasive breast carcinoma cells. Such a feedback loop cannot explain the evident stability of tissue-specific ratios of macrophages to stromal cells in the tissue microenvironment, nor how adaptive physiologic demands for macrophage functions or macrophage-produced growth factors affect the local MCSF production set point. We hypothesize that macrophages produce a signal reporting their MCSF requirements to stromal cells, and that stromal cells respond to this signal by inducing CSF1 transcription, inhibiting macrophage signal production once through feedback inhibition, and postulate that a reciprocal negative feedback loop exists reporting stromal cell demand for macrophage functions or growth factors. Consistent with our hypothesis, our preliminary data in a mouse embryonic fibroblast - macrophage co-culture system suggests that stromal cell CSF1 expression may be induced significantly by as yet unknown intercellular signals, and conditioned media transfer experiments suggest that this CSF1-inducing signal may be mediated by a soluble factor. Furthermore, little is known of how a functional mismatch between MCSF production by tumors and tumor- associated macrophage (TAM) MCSF requirements leads to tumor pathology. Although some tumors are known to overexpress CSF1 successfully to recruit TAMs, we hypothesize that tumors fail to adequately regulate local MCSF production to TAM demands, leading to increased necrosis, hypoxia, pro-inflammatory TAM and tumor cell death. In this project, we combine in vitro macrophage and stromal cell co-culture systems, transcriptional analysis, with biophysical and biochemical methods for molecular signal isolation. Additionally, we develop in vivo models of CSF1-attenuated tumor cell lines and carcinogen-induced hepatocellular carcinoma on a liver-specific CSF1 conditional knockout model to test these hypotheses, to together better elucidate the CSF1-dependent signaling mechanisms between macrophages and stromal cells in normal tissue homeostasis and tumorigenesis.